If you're using hydroponics to grow, the anchoring medium shouldn't make a great deal of difference. Roots will be immersed in the nutrient solution so they'll be getting what you serve them. Perlite, gravel, or even especially tailored sponges will all be adequate for holding the roots down into the solution. I hope you'll be eating the mustard plant leaves, and not expecting the plant to flower and produce mustard seeds. Just treat it as any tender green crop (like lettuce), and give it 40-60 days to mature.